In manufacturing of a seamless pipe in accordance with a Mannesmann mandrel mill method, a hollow shell is manufactured by first of all heating a round billet or a rectangular billet by a heating furnace, and thereafter piercing and rolling by a piecer. Next, a mandrel bar is inserted to an inner surface of the hollow shell and is drawn and rolled by a mandrel mill constructed by a plurality of rolling stands. Thereafter, a product is obtained by forming and rolling the pipe material to a predetermined outer diameter by a sizing mill.
Conventionally, as shown in FIG. 1A, there has been used a 2-roll type mandrel mill in which two opposing grooved rolls R11′ and R12′ are arranged in each of rolling stands, and are alternately arranged in such a manner as to shift pressing directions of the grooved rolls R11′ and R12′ at 90 degrees between the adjacent rolling stands. Further, as shown in FIG. 1B, there has been used a 3-roll type mandrel mill in which three grooved rolls R21′, R22′ and R23′ are arranged in each of the rolling stands in such a manner that an angle formed by pressing directions of any two adjacent grooved rolls of the three grooved rolls R21′, R22′ and R23′ comes to 120 degrees, and are alternately arranged in such a manner as to shift pressing directions of the grooved rolls R21′, R22′ and R23′ at 60 degrees between the adjacent rolling stands. Further, as shown in FIG. 1C, there has been applied a 4-roll type mandrel mill in which four grooved rolls R31′, R32′, R33′ and R34′ are arranged in each of the rolling stands in such a manner that an angle formed by pressing directions of any two adjacent grooved rolls of the four grooved rolls R31′, R32′, R33′ and R34′ comes to 90 degrees.
In this case, in order to secure a thickness precision of the material to be rolled in the mandrel mill, and suppress a thickness deviation, it is important to set a pressing position of each of the grooved rolls (position of each of the grooved rolls with respect to the material to be rolled at a time of rolling the material to be rolled) provided in each of the rolling stands of the mandrel mill to a proper position. Specifically, as shown in FIGS. 1A to 1C, it is important that a groove bottom B of each of the grooved rolls comes to a position which comes away evenly at a desired amount from a center O of a pass line of the material to be rolled. However, due to a dimensional tolerance, an installation error and the like of each of the grooved rolls and a tool holding the grooved roll, it is actually hard to set the pressing position of each of the grooved rolls in accordance with a design value.
Accordingly, in the 2-roll type mandrel mill, there is used a method of moving the opposing grooved rolls R11′ and R12′ in the pressing direction (direction of an arrow in FIG. 1A), bringing flange portions F′ into contact with each other so as to press to each other at certain load, and regulating the pressing position in the pressing direction by setting the positions of the respective grooved rolls R11′ and R12′ at this time to reference positions in the pressing direction. Specifically, after the reference position of each of the grooved rolls R11′ and R12′ is decided, the position of each of the grooved rolls R11′ and R12′ is evenly moved in the pressing direction from the reference position.
However, in the case of the 3-roll type or 4-roll type mandrel mill, since a degree of freedom of a relative position between the positions of the respective grooved rolls is great, it is not possible to suitably decide the reference position in the pressing direction of the grooved roll by the method in the case of the 2-roll type mandrel mill mentioned above. Accordingly, since it is not possible to regulate the pressing position of each of the grooved rolls to the proper position, there is a problem that it is hard to suppress the thickness deviation of the material to be rolled.
In Japanese Unexamined Patent Publication No. 2005-131706, there has been proposed a method of arranging a thickness measuring apparatus in an outlet side of the mandrel and regulating the pressing position in the pressing direction of each of the grooved rolls based on a thickness measured value of the material to be rolled measured by the thickness measuring apparatus, in the 3-roll type mandrel mill. However, since a measured value by the thickness measuring apparatus does not exists, with regard to the material to be rolled which is first rolled, it is not possible to regulate the pressing position of each of the grooved rolls to a proper position, at least with regard to the first material to be rolled, and it is hard to suppress the thickness deviation.
On the other hand, even in the 2-roll type mandrel mill, there is a case that positions of the grooved rolls R11′ and R12′ in a direction (direction shown by an arrow in FIG. 2) which is vertical to the pressing direction of the grooved rolls R11′ and R12′ are shifted, as shown in FIG. 2, due to the dimensional tolerance, the installation error and the like of each of the grooved rolls and the tool holding the grooved roll. If the displacement in the direction which is vertical to the pressing direction is generated, the thickness deviation is generated in the material to be rolled P. However, the displacement cannot be set right by the method of moving the grooved rolls R11′ and R12′ in the pressing direction so as to bring the flange portions into contact with each other.
In Japanese Unexamined Patent Publication No. 2003-220403, there has been proposed a method of individually regulating a closing amount in each of the flange sides of the grooved rolls provided in the mandrel mill, based on a thickness measured value of the material to be rolled measured in a downstream side of the mandrel mill. In accordance with the method described in Japanese Unexamined Patent Publication No. 2003-220403, it is possible to regulate the pressing position of the grooved roll even in a direction which is vertical to the pressing direction, by differentiating the closing amount in each of the flange sides. However, since the thickness measured value does not exist with regard to the material to be rolled which is first rolled, it is not possible to regulate the pressing position in the direction which is vertical to the pressing direction of each of the grooved rolls to the proper position, with regard to at least the first material to be rolled, and it is hard to suppress the thickness deviation as shown in FIG. 2. This is the same in the case of the 3-roll type and 4-roll type mandrel mills.
The problem of the prior art mentioned above is not limited to the mandrel mill, but is in common to the rolling stand rolling the material to be rolled by using the grooved roll.